A/C reached critical mass!

[engineer_gregh]

Stock setup on my big block. When the engine was out and rebuilt 4 years ago I replaced the expansion valve, eliminated the POA valve and installed a High Lo pressure switch, new receiver dryer, vacuumed and converted to 134a. Its always been ok but not great. Its been working for the past three years but cycled continuously at anything above idle. I decided that I would adjust the pressure switch. I upped the idle in park to try and duplicate the cycling; I'm guessing the head pressure reached a point that literally blew the manifold line right off the back of the compressor. I'm not sure why the A/C didn't cycle in park like it would with the vehicle moving? I'm open for opinions on the cycling and if I should just rip out the factory setup and go with vintage air? I don't even know if I can just bolt the hose back on the back of the compressor? I think it (hose with the manifold) is held on by a single bolt in the center but I've not found the bolt since the explosion.

[davepl]

I wouldn't replace the whole system just because something is wrong with it, that's an expensive troubleshooting approach!

I don't know anything about removing the POA, but when not leaking mine works fine with all stock components except a parallel-flow condenser.

[Eddie H.]

I would recommend putting a POA valve back on it since that is how the system was designed to work originally. (It needs to be adjusted to work properly with R134.) I'm not a big fan of the POA eliminator setup. Replace the drier, all the o rings, flush the system, and evacuate and charge the system properly and you should be good to go.

[F.A.A.benny]

Id go back to a poa valve recalibrated for 134a. Its easier on the A6 compressor which wasn't designed to cycle. And replace the factory tube and fine condenser with a parallel flow condenser. Those are the two issues that a lot of people don't do when converting to 134a

[leddzepp]

I would keep the factory system and put the POA back. Then go back to R-12 which is what the system was designed for and enjoy a "fantastic" system that blows snow no matter how hot it is, instead of an "ok" system with problems.

[F.A.A.benny]

Originally Posted by leddzepp

I would keep the factory system and put the POA back. Then go back to R-12 which is what the system was designed for and enjoy a "fantastic" system that blows snow no matter how hot it is, instead of an "ok" system with problems.

That is actually the simplest thing to do also. r12 can be had on CL, swap meets and ebay. Its not really that expensive either.

[leddzepp]

Originally Posted by F.A.A.benny

That is actually the simplest thing to do also. r12 can be had on CL, swap meets and ebay. Its not really that expensive either.

Exactly. I bought 5 cans at the Pomona swap meet recently for $15 a can. That is actually less than r-134 here . I don't get the resistance to using it. It isn't like gasoline, you don't keep refueling

[buddylee]

The POA is designed and installed to prevent freeze ups in the evaporator. Without one in the system, your compressor should have been working full speed at all times and the car should have been ice cold without any way to "throttle " the cooling.
The pressures of R12 and R 134a are fairly close. Why didn't the high pressure switch shut the compressor off? Is the condenser dirty? While idling there couldn't have been perfect airflow. Can you show a picture of where the leak occurred? It's possible that there was air/non condensables in the system that caused the pressure spike
I would install a POA, change dryer, and pressurize with nitrogen before evacuating and installing R134a.

[Andy4639]

I'm going the other direction. Put a acumaltor in it and new hoses and 134a and never look back.
This is a older picture but it shows the system. Over 20 years with no problems operating.

[davepl]

Originally Posted by leddzepp

Exactly. I bought 5 cans at the Pomona swap meet recently for $15 a can. That is actually less than r-134 here . I don't get the resistance to using it. It isn't like gasoline, you don't keep refueling

But you do! Until you find all the leaks, and I'm not there yet. If I had taken the almost-universal admonition to use R-12, I would have leaked it all out by now...

I can see using R-134 until you know it's solid and then using R-12, but it worked well enough (for me with a parallel condenser) with R-134 that I then wouldn't make the switch.

So I guess all I'm saying is don't waste you're R-12 unless you're confident it won't leak. Mine vacuums OK for hours but won't hold overnight.

[engineer_gregh]

Thanks for the responses everyone. The system has worked as configured for the past 3 years, just cycled continuously at anything above idle. IMO the reason the thing was cycling was that the pressure was going too high and the switch was doing what it was supposed to do. I screwed up by shorting across the pressure switch so that I could adjust the set screw. By shorting across the switch I actually prevented the switch from working and when I got to about 2K RPM's, BOOM, it blew the line off. So if in fact the pressure was too high, there has to be blockage some where. I'm going replace the original condenser, dryer, pull a good vacuum, and oil and recharge the system. The compressor is a remanufactured 4 seasons. My guess is that the original compressor failed, filled the condenser with crap and flushing it just won't work. Of course I'm assuming that I can put another screw in the line and get it to tighten up to the back of the compressor. I may have to replace the line and compressor if it screwed both of them up. Some times I just do stupid crap!

[F.A.A.benny]

I personally wouldn't charge a system that wouldn't hold a vacuum overnight. If the leak cant be found easily, charge the system with nitrogen and using soapy water will locate the leak.

[davepl]

Originally Posted by F.A.A.benny

I personally wouldn't charge a system that wouldn't hold a vacuum overnight. If the leak cant be found easily, charge the system with nitrogen and using soapy water will locate the leak.

That's next! Actually I'm going to have the compressor replaced because it's slinging a ton of oil off its snout, and then have them leak-check it.

Anyone know if you can get at the evaporator core from inside the truck, or does the firewall need to come apart?

[Andy4639]

The evaporator is on the motor side of the box. The heater core is on the inside.

[davepl]

Thanks. From cursory inspection it looks like you should be able to get at it by taking the whole underhood box apart in the middle, correct?

Does anyone know? I'm just worried I don't want some shop I have little experience with pulling fenders off my truck, etc...

I have to use an "unknown to me but otherwise recommended" shop because my preferred "old men that fix cars" shop doesn't have the AC equipment. Nor do I, or I'd do it myself! I could just vent all the R134 but that's probably illegal, might be bad for the ozone, and is expensive.

[F.A.A.benny]

The evap housing separates and the drivers side is removed. There are 8 screws plus the hardware that holds the poa and expansion valves.

[engineer_gregh]

Pictures

[engineer_gregh]

more

[engineer_gregh]

Now with the clamshell and evaporator removed you can see that at least half of the evaporator was covered in years of leaves and dirt. Benny has posted before that you should remove the evaporator to clean it, I heeded his advise. After removal I've used two cans of brake cleaner to flush and its still not pouring out clean. I'll post more pictures later.

[engineer_gregh]

Update; pictures of Old Air Products condenser. I placed the new one on top of the original condenser so it would be easier to see size and comparison of the two.

[engineer_gregh]

So today I started the A/C tear down. I bought a POA valve calibrated for 134a from F.A.A. benny. He was kind enough to take my call and answer a lot of questions before I started this process. I thought I would pay it forward and post pictures of the tear down and share what money I've spent rebuilding the system. I'm replacing the original condenser and main hose with part#11-7201 $220 and part#95-7215A $115; shipping costs $49 from Old Air products. I bought a reman compressor from Auto Zone for $134.99 and a new receiver dryer for $16.99.

[engineer_gregh]

My goal in posting the pictures is that you can see what screws need to be removed and just how clean the evaporator core looked from the outside.

[clay68c10]

Nice work, I have the same POA system in my 68 caddy and had to play around with the fill level. 80% was too much, the hi side was way up there.
Like you, I did somewhere around 60-70% and it works well.
Your idling conditions will mimic what it's like to be stuck in traffic, so I'd see how it does.

[Andy4639]

Nice write up on your system and what you did.

One thing a lot of people don't understand about AC is that the more air you move across the condenser at any given time is how you get colder air into the cab.
So if your sitting in your garage idling and you get down to 40* you are as good as you need it. If you start dipping into the 30's then it's going to freeze up on you. 38* to 40* is where I say you need to be and it will be fine once your going down the high way.
This is why I like to run a fan off of the AC switch. To get the most air effect that I can. This is the reason you see vehicles ac temps go up sitting at a light. My truck can sit for a long time with the ac running and it stays right at 190* water temp and AC blowing at 38* to 40* no matter what the outside temp is.

I also run my AC fan through a micro switch mounted out on the front of the grille to cut the fan off at road speed which is 25MPH. This gives me longer fan life and it's not needed when the vehicle is moving.
Another small thing is sealing the radiator off forcing the air to go through it instead of around it.
225 to 250 is what you should have on the high side gauge at 85* air temp.45-55 PSI on the low side at 85* give are take a little either way.Get it close and see how it sweets if it's sweeting good and making a nice puddle of water leave it alone.

[engineer_gregh]

Thanks Clay. Its still not right. The high side pressure is going above 300 at idle. I'm going to replace the POA and expansion valve. I'll update once thats done, probably be a week or so. Nothing is ever easy on this truck:-)